This invention relates to an apparatus for accessing a target position on a tape along a transportation path thereof in a tape recorder, and more particularly to a tape location control apparatus for a tape recorder used for determination of head position at the time of editing, etc. using a digital audio tape recorder, or a video tape recorder, etc. for professional use in preparing a master tape.
In recent years, in preparing a master tape for producing optical digital audio discs, compact discs, etc., or digital audio software such as audio tapes, etc., there are many instances where digital recording is carried out. Many processes from the recording process up to the cutting process are digitally carried out. The equipment provided for professional use must provide high accuracy and capabilities. For example, the capability of accessing a desired target position or location at high velocity leads to reduced editing time. This is therefore advantageous from the viewpoint of cost. High velocity access to a preselected location on a tape is carried out by a tape position control, or so called locate control.
In order to implement the tape position control (locate control) an auxiliary track is employed wherein, for example, tape location information is recorded so that by reproduction of the auxiliary track the tape location may be detected. The use of so called control (CTL) track or a so called time code (TC) track, etc., as such an auxiliary track, is known. Further, an approach is also known in which the revolution of a reel table, a tape contact roller, or so called timer roller, etc. is measured to detect the tape location. Known operational modes for carrying out the tape location function using tape location information detected by these methods include a control (CTL) mode making use of a sector address on the control track, a timer mode using a count value of so called a timer roller, a time code (TC) mode using a time code on the tame code track, and the like.
The outline of the configuration of a tape running and driving system including a servo system for tape location control will now be described with reference to FIG. 5.
In FIG. 5, reel sensors 1A and 1B first detect the diameter of a reel of a tape and send the diameter thus detected to a tape running system control unit 2. In addition, FG signals from FG (frequency generation) signal detectors 7A to 7C for respectively detecting the revolutions of reels 5A and 5B, and capstan rollers 6A and 6B, control data or time code data detected at a reproducing magnetic had 8, and a mode switching signal, etc. are delivered to the control unit 2. The tape running system control unit 2 carries out an internal process based on these signals to send a clutch ON/OFF control signal or a FG/CTL servo signal, etc. to a capstan servo circuit 3. Responding to these signals, the capstan servo circuit 3 drives and controls revolution of the capstan rollers 6A and 6B. Further, the tape running system control unit 2 sends, to a reel servo circuit 4, an ON/OFF control signal for the velocity servo, a velocity servo gain adjustment signal, an ON/OFF control signal for the tape position control gain, a tension setting signal, and an indication voltage signal, etc. Thus, the reel servo circuit 4 controls motor drivers 9A and 9B to drive and control revolution of the respective reel motors. In addition, a detected tension value from a tension adjustment unit 10 is fed back to the reel servo circuit 4 and is input thereto.
Further, the reel servo circuit 4 determines a difference between the detected tape current position and a target tape position to be accessed, referred to as an address difference (ADR-DIF) to drive and control the reel motors so that the tape running velocity varies in accordance with a velocity determined in advance in correspondence with the address difference (ADR-DIF) and referred to as a target indication velocity. The above-mentioned target indication velocity (TGV) in this case has a distance-velocity characteristic curve such that, e.g., the tape running velocity has a high value at a position relatively far from the target tape location, the velocity has a lower value as the target position approaches and the tape is stopped at the target location. Thus, the tape is allowed to smoothly approach the target tape location at a high velocity. In this case, the steps of inputting data indicative of the address difference (ADR-DIF) to a predetermined target indication velocity table ROM, etc., reading out a correspondence target indication velocity (TGV-LOC), adding/subtracting an acceleration rate during each predetermined sampling period, e.g., 10 m sec., and thereafter providing a limit at a limit velocity, are carried out by digital signal processing. The output data is treated as an actual indication velocity (CUR-TGV), and is sent through a D/A converter, etc. to an analog servo circuit unit having a velocity servo loop as a minor loop, at which a servo operation is carried out so that any error (velocity deviation) between the actual tape velocity (actual velocity) and the above-mentioned indication velocity data (CUR-TGV) becomes equal to zero.
A locate control curve relating the target indication velocity (TGV-LDC) to the address difference (ADR-DIF)) set in the above-mentioned ROM table, etc. is determined in advance by calculation based on the maximum velocity or the maximum acceleration rate of the reel servo system, etc. In actual terms, a calculation is performed, e.g., on the assumption that the maximum velocity is 16 m/s, and the maximum acceleration rate is about 8 m/s.sup.2 in the case of a 10 inch reel and about 3 m/s.sup.2 in the case of a 14 inch reel.
Meanwhile, in implementing a tape location control (locate control) as described above, since the loop gain of the velocity servo/tension servo cannot become infinite in reality, the response cannot always follow. For example, in the case of a ramp response, the velocity deviation cannot become zero. Namely, a velocity deviation proportional to the time constant and the acceleration, and inversely proportional to the gain necessarily exists. For this reason, the actual velocity with respect to the indication velocity (CUR-TGV) of the reel servo is increased by the magnitude of the velocity deviation. As the acceleration rate becomes larger, the deviation increases accordingly. Therefore, when, e.g., the case of the 10 inch reel (acceleration rate 8 m/s.sup.2) is compared with the case of the 14 inch reel (the acceleration rate 3 m/s.sup.2), the velocity deviation of the 10 inch reel is 8/3 times larger than that of the 14 inch real. Even if the operation is performed at the same indication speed, the respective acceleration rates vary by the difference between inertial moments due to the difference in reel diameters.
Further, in the previously described location control technique, since the sampling in the TC mode is more coarse than that in the CTL mode, there occurs a time delay in the TC mode which is about 30 to 42 times larger than that of the CTL mode.
Namely, in the CTL mode, due to the above-described velocity deviation, as shown in FIG. 6, there occurs an error between the curve b representing the target indication velocity data (TGV-LOC) and the curve a representing the indication velocity data (CUR-TGV) for the reel servo. In the TC mode, although the velocity deviation is the same as in the case of the CTL mode, the difference between the curve C representing the target indication velocity data (TGV-LOC) and the curve a representing the indication velocity data (CUR-TGV) in the TC mode is further increased, as shown in FIG. 6 due to the time delay corresponding to the sampling time.
In either mode, because of velocity deviation with respect to the actual velocity and the time delay due to sampling, the acceleration rate of the target indication velocity (TGV-LOC) will become larger than 8 m/s.sup.2 in the case of the 10 inch reel. As a result, the indication velocity fails to follow.
In the prior art, since it is not recognized that a deviation occurs between the target indication velocity (TGV-LOC) and the indication velocity (CUR-TGV), particularly in the case of the locate control in the TC mode, when an extraordinary state appears such as an unstable operation of the reel servo, the velocity servo gain is severely adjusted to cope with this event. However, such adjustment is difficult, and cannot be sufficiently made for an unstable operation including a different between accelerations for different reel diameters. In the case where an adjustment is not made such that the velocity servo gain falls within the range of .+-.1 dB, an effect like overshooting results.